Three-point hitch mechanisms of the type used on tractors for attachment of various trailing implements frequently require adjustment in the lengths of one or more of their arms to accommodate a particular implement or to adjust for varying operating conditions. Three-point hitch mechanisms typically include an upper link arm which is adjustable in length.
Many different rigid arms with adjustable lengths have been developed over the years for tractor hitch mechanisms and the like. Among such prior hitches and adjustable arms are those disclosed in the following United States patents:
U.S. Pat. No. 4,418,935 (O'Connor) PA1 U.S. Pat. No. 4,034,999 (Jackson) PA1 U.S. Pat. No. 3,053,552 (Horney) PA1 U.S. Pat. No. 2,878,043 (Edman et al.) PA1 U.S. Pat. No. 2,920,912 (Hess) PA1 U.S. Pat. No. 4,194,757 (Lucas et al.) PA1 U.S. Pat. No. 2,998,989 (Silver et al.) PA1 U.S. Pat. No. 3,989,394 (Ellis) PA1 U.S. Pat. No. 1,245,095 (Flood) PA1 U.S. Pat. No. 3,851,978 (Kuipers) PA1 U.S. Pat. No. 4,418,583 (Taig)
The adjustable arms and hitches of the prior art have certain problems and disadvantages. Certain adjustable arms generally of the turnbuckle type, that is, having right-hand and left-hand threaded end members coaxially joined to a rotatable middle member, have inadequate means for locking after adjustment in length. In some cases, only one of the threaded engagements of an end member to the middle member is locked, or separate locking mechanisms are required for the two threaded engagements. Complex locking devices are often used and multiple locking steps are often required.
In certain situations, one or more elements of a locking structure extend well beyond at least one of the ends of the middle member in order to engage something which serves as an anchor against rotational movement.
The operation of some devices may not only be complex in structure and operation, but difficult to understand. In many cases operators may have to resort to their tool chests when adjusting the lengths of such adjustable arms. This may be necessary for turning the middle member or for unlocking or locking it.
There is a need for an improved multi-arm hitch apparatus and for an improved adjustment and locking means for rigid link arms of adjustable length. In particular, there is a need for a simple adjustment means to firmly lock both end portions and the middle portion of turnbuckle-type adjustable arms using a single locking piece and single locking movement. There is a need for such an adjustment means which is self-contained, that is, free of locking structures extending beyond the ends of the middle member. There is a need for a link arm with these advantages which may be easily adjusted by hand, without using tools.